Method for producing prestressed concrete

ABSTRACT

The invention consists of an improved method for producing prestressed concrete products wherein the improvement comprises the utilization of the concrete itself to provide the prestressing force which must be exerted on reinforcing wire strands used in the manufacture of said prestressed concrete products. The reinforcing wire is fed into and attached to the first mold in a train of molds and then through each of the following molds in series. The molds have fluent concrete inserted therein as they are guided along an endless track to a point at which the concrete has hardened and whence said molds move by force of gravity down an incline. The hardened concrete in the initial molds descends said incline at a computercontrolled rate whereby a calculated stress is imparted to the reinforcing wire strands in the subsequent molds containing fluent concrete. The calculations are based upon the force supplied by the weight of the descending molds containing hardened concrete and the rate at which said hardening occurs. The stress upon said reinforcing wire is gradually and evenly transferred from the wire to the concrete as the molds descend the incline whereupon, when the train of molds reaches a point at which the calculated stress is nulled, said reinforcing wire strands are severed at the point of entrance into said mold. A continuous production cycle may thereby be maintained with the subject invention whereby an economic advantage is obtained over prior art techniques.

United States Patent [191 Brown, Jr.

[4 1 Sept. 2, 1975 METHOD FOR PRODUCING PRESTRESSED CONCRETE [76]Inventor: Patrick F. Brown, Jr., 113

Columbia Dr., Oak Ridge, Tenn. 37830 22 Filed: Apr.1l, 1974 [21] Appl.No.: 460,062

[52] US. Cl. 264/40; 264/228', 264/297; 264/333; 425/111 [51] Int. Cl.B28B 23/04 [58] Field of Search 425/1 1 l; 264/40, 228, 229, 264/297,333

[56] References Cited UNITED STATES PATENTS 2,394,227 2/1946 Barber264/228 X 2,569,231 9/1951 Danhof 264/228 X 2,607,099 8/1952 Schroder eta1. 264/228 X 2,745,164 5/1956 Ros 264/228 X 2,863,206 12/1958 Kirchner264/228 X 3,055,073 9/1962 Gerwick, .lr. 264/228 X 3,283,385 11/1966Oakden 264/228 X 3,694,118 9/1972 Crowder 264/228 X FOREIGN PATENTS ORAPPLICATIONS 6/1959 United Kingdom 264/228 Primary E.ramt'nerRobert F.White Assistant E.\-aminer-Thomas P. Pavelko Attorney, Agent, orFirmHerman L. Holsopple 5 7 ABSTRACT The invention consists of animproved method for producing prestressed concrete products wherein theimprovement comprises the utilization of the concrete itself to providethe prestressing force which must be exerted on reinforcing wire strandsused in the manufacture of said prestressed concrete products. Thereinforcing wire is fed into and attached to the first mold in a trainof molds and then through each of the following molds in series. Themolds have fluent concrete inserted therein as they are guided along anendless track to a point at which the concrete has hardened and whencesaid molds move by force of gravity down an incline. The hardenedconcrete in the initial molds descends said incline at acomputer-controlled rate whereby a calculated stress is imparted to thereinforcing wire strands in the subsequent molds containing fluentconcrete. The calculations are based upon the force supplied by theweight of the descending molds containing hardened concrete and the rateat which said hardening occurs. The stress upon said reinforcing wire isgradually and evenly transferred from the wire to the concrete as themolds descend theincline whereupon, when the train of molds reaches apoint at which the calculated stress is nulled, said reinforcing wirestrands are severed at the point of entrance into said mold. Acontinuous production cycle may thereby be maintained with the subjectinvention whereby an economic advantage is obtained over prior arttechniques.

9 Claims, 2 Drawing Figures METHOD FOR PRODUCING PRESTRESSED CONCRETEBACKGROUND OF TI-IE INVENTION The invention described herein relates toprestressed concrete and, in particular, to a method for prestressingreinforcing wire for the continuous production of prestressed concrete.It is an improvement of known methods for forming prestressed concretearticles such as that described in U.S. Pat. Nos. 2,863,206, 3,055,073,3,694,118, 3,283,385 and 2,607,099.

In the production of prestressed concrete it is customary to feedpretensioned wires into a fixed mold after which fluent concrete isdeposited onto and around thewires. Tension is then placed on the wiresby means of jacks. After the concrete hardens the highly tensioned wiresare severed at their point of entrance'to themold whereupon the stressexerted within the wires is imparted to the concrete. The individualcontents of each mold are then removed and the process can then berepeated.

It is not a novel idea to produce continuously prestressed concrete asthe alternative to individual production of concrete members. Numerouspatents exist for such procedures. Known processes also exist forproviding tension upon the reinforcing wires used-in manufacturingprestressed'concrete. These processes depend upon various gripping,grasping or pulling means for engaging the wire and exerting thenecessary tension. However, no process is known wherein advantage istaken of the mass and weight of the concrete itself together with theweight of the molds to exert tension upon said reinforcing wire and tothereby contribute to the continuous production of prestressed concretemembers as is herein described.

SUMMARY OF THE INVENTION It is the object of this invention to providean improved method for the mass production of prestressed concretearticles which utilizes a moving train of mold forms on an endlesstrack, said track having an inclined portion included therein wherebysuch articles as railroad ties, fence posts and small beams may beproduced at low unit cost and whereby a gradual transference of stressesfrom the wire strands to the hardened concrete is accomplished andwherein a gradual reduction of stresses is made upon the wire betweenmold forms subsequent to the hardening of said concrete and prior to theremoval of said articles from theirmolds.

It is another object of this invention to provide an ec onomical methodfor the continuous production of prestressed concrete products whereinthe required prestressing force for such production is supplied by themold train and the concrete products therein.

It is an additional object of this invention to provide an improvedmethod for exerting tension on reinforcing wire used to continuouslymanufacture prestressed concrete products which does not depend uponcomplicated gripping, grasping or pulling means for the engagement ofthe wire.

It is a still further object of this invention to provide an improvedmethod for the continuous manufacture of prestressed concrete productswhich will utilize the topography of hilly or mountainous terrain andwhich will thereby minimize the detrimental effe cts on the en vironmentwhich would occur if said terrain was required to be altered.

Further features and objects of the invention will be apparent fromanexamination of the accompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT In general my invention relates to a procedure for thecontinuous manufacture of prestressed concrete products whereinreinforcing wire strands are continuously fed from wire supply reels,through a-braki'ng mecha nism, then attached within the confines-of thefirst mold in a train of moving mold forms; fluent concrete is pouredtherein as the molds are guided along a generally level portion of anendless track to a point at which the concrete becomes hard and whencethe molds move by force of gravity down an incline. The hardenedconcrete in the initial molds descends the incline at acomputer-controlled rate whereby a calculated stress is imparted to thereinforcing wire strands in the following molds containing fluentconcrete. Said calculations are based upon the tension imparted by theweight of the descending molds and the rate at which the concretehardens. The latent tension stored within each of the stressedreinforcing wire strands is gradually transferred to the hardenedconcrete as the molds descend the incline whereupon, when the train ofmold forms reaches a point at which the calculated stress is nulled,said reinforcing wire strands are severed thereby providingsaid concretewith an effective tensile strength considerably greater than thatoriginally present.

Conventional means are used to sever the reinforcing wire strands suchas by cutting said wires with wire cutters, high speed saws or by meansof gas or electric torches. Severance of the molds should take placeonly after the first mold in the train of molds has reached a horizontaland level portion of the endless track subsequent to reaching the end ofthe incline or at a point wherein retarders of the type used in railroadhumping operations have offset the pulling effect of the moving mold. Atthis point the first mold in the train of molds is no longer exertingany significant stress on the following molds and the tension which hadgradually been imparted to the reinforcing wire strands will essentiallyall be transferred to the hardened concrete. The concrete products maythen be disengaged from their molds by conventional means.

I-Iaving reference to the drawings in detail, 10 represents acontinuously moving train of mold forms 12, 12 moving horizontally inthe direction of arrow 14. Mold forms 12 are designated as thosecontaining fluent concrete while mold forms 12 will be considered ashaving hardened concrete therein. The mold forms 12, 12' can comprise aplurality of shapes according to the purpose for which they may be usedas for bridge girders, roof slabs, fence pos ts," railroad ties,structural columns, beams, power poles and the like.

In the preferred embodiment, mold forms 12, 12' are mounted on separatecarriages having flanged wheels 16 whereby mold forms 12, 12 can beguided along endless track 18. Any conventional method may be used toinitiate movement of the mold forms 12, 12' along track 18 such as bymeans of endless cog chain 19 or the mold forms 12, 12' may be towed orseparately self-propelled by means of an electrical, gasoline, diesel orbattery powered engine or track 18 may have a very slight inclineleading to the position shown at A whereby friction of flanged wheels 16on track 18 is offset. It is an important feature of this invention thatit is the weight of the molds and their contents which provides therequired stress upon the reinforcing wire strands. No propelling force,other than gravity, is therefore required for movement of the train onceit has started down the incline.

As the moving train of molds l progresses horizontally in the: directionof arrow 14 a plurality of reels 20 havingreinforcing-wire strands 21wound thereon feed said reinforcing wire strands 21 into a conventionalwire control device 22 such as an electrical regenerative brake wherebybraking action is applied to reinforcing wire strands 21 relative to therate at which mold forms 12, 12 are moving along track 18. Saidreinforcing wire strands 21 are physically attached to the first mold inthe train of molds and said wire is thence fed in series through each ofthe following mold forms 12.

Fluent concrete is thereupon admitted to each mold form 12 from concretehopper 23. It is important in the operation of this invention that theconcrete admitted from hopper 23 be set sufficiently to accept transferof stress from the reinforcing wire strands 21 at the time the moldforms 12 reach position A of FIG. 1. In present commercial practice mostof the wires tension is transferred within 24 hours. Thus, in thepreferred embodiment, a time span of approximately 1 day would occupythe movement of mold forms 12 from concrete hopper 23 toposition A.

In order to insure that mold forms 12 will proceed at a constant spacingduring the concrete hardening process and to prevent longitudinalmovement of wire strands 21 relative to the fluent concrete in the moldforms, at least one adjustable spacer bar 24 is secured and locked inplace between mold forms 12 containing said fluent concrete. It shouldbe noted that the mechanism for maintaining a constant spacing betweenmold forms relative to each other is not limited to use of spacer bar24. It is obvious that numerous known releasable clamping devices areavailable for performing the function of maintaining said constantspacing between the mold forms. Spacer bar 24 or a similar appropriateconstant spacing or clamping means is retained between said mold forms12 only until the fluent concrete has hardened and the moving train ofmold forms begins the descent at position A whereupon said constantspacing or clamping means is removed and the full weight of the movingtrain of mold forms 12' is ap plied to reinforcing wire strands 21.

While not required in the practice of this invention, it is obvious thata heated curing tunnel 26 through which mold forms 12 pass prior totheir descent on incline AB would greatly decrease the time required forthe concrete to harden and therefore could'be well justified on economicgrounds. i

As shown in FIG. 1, when mold forms 12 reach position A said forms begina descent covering the incline AB, said incline terminating at B wheretrack 18 again becomes generally level. The weight of mold forms 12 andthe contents thereof provides tension upon reinforcing wire strands 21in mold forms 12 having fluent concrete therein and thereby providestheprestressing force required for the subject process.

In the preferred embodiment, the inclined portion AB of track 18 shouldhave a pitch of between and 60 from the horizontal and should be of sucha length as to provide the calculated force and stress which isrequired. This force is determined primarily by the mass and weight ofthe mold forms employed, the concrete therein and the rate at whichhardening occurs and is computer-controlled whereby retarding means 27can be automatically applied to prevent'overstressing of the reinforcingwire strands 21 and thereby maintain a constant preset tension upon saidreinforcing wire strands.

Braking retarders 27 of the type used in railroad humping operations canbe conveniently used at any point on incline AB to automatically reducethe stress on the wire. It is an important feature of this inventionthat tension is gradually transferred to the hardened concrete. Thus, asmold forms 12' arrive at position B, track 18 is again generally leveland the tension exerted by gravitational forces is correspondinglyreduced. It is, therefore, seen that tension has been applied in agradual manner by slowly increasing the load on the wire strands 21imbedded in fluent concrete until a calculated constant tension isachieved and subsequently, when the concrete has hardened, tension isthen reduced in a gradual way, either by the use of braking retarders 27or by the arrival of the mold forms 12' to a generally level portion oftrack 18 at B whereby the gravitational load is lessened.

Control of tension is maintained by a conventional computer programmedto convert the tensionproduced data derived from the effect ofgravitational forces on the weight and mass of the concrete and moldforms moving down the inclined zone AB into the braking action ofretarders 27 whereby a programmed reduction of stresses is obtainedaccording to the size and strength requirements of the particulararticle being manufactured. It is obvious that conventional concretestress tables are of use in performing the calculations which arerequired prior toprogramming a computer for controlling the moving trainof mold forms of the subject invention. When tension and/or the load isreduced in wire strands 21 in the aforedescribed manner of thisinvention said wire strands 21 may be severed by any conventional meanswhereby a greater percentage of prestressed force is transferred to theconcrete than has heretofore been possible. I

In F IG. 2 is shown a simple and direct method to provide for continuousproduction of prestressed concrete articles. As the continuously movingtrain of mold forms 10 moves along endless track 18, tension is suppliedto reinforcing wire strands 21 in the fluent concrete in mold forms 12by means of the weight and mass of hardened concrete in mold forms 12'.When the moving train of mold forms 10 reaches concrete removal station28 the prestressed concrete products are removed from the mold forms byconventional means suchas by the use of conventional hoist cranes or bythe use of mold for ms rotatable along their horizontal axis therebyenabling the product tobe rolled onto a loading platform or loadingconveyor. Empty mold forms then proceed under their own power or bymeans of cog chain. l9from concrete removal station 28 to the point atwhich wire strands 21 and fluent, concrete from concrete hopper 23 areadmitted to begin the cycle anew.

It is seen from the foregoing drawings and specifications that myinvention can be utilized to continuously produce prestressed concreteproducts with none of the complicated grasping, gripping or pullingequipment common to prior art tension producing techniques. It isfurther seen that my invention will make use of steep terrain such as iscommonly found in the hilly Appalachian Mountains or other mountainousregions of the United States and which invention can contribute to theindustrial development thereof but which invention is not limitedthereto since the inclined track of this invention can be installed anderected in many and varied locations.

It will be understood that variations of the described procedure may beemployed without departing from the scope of the invention. For example,instead of using an inclined plane down which the mold forms proceed, asufficiently heavy weight can be attached to the reinforcing wirestrands and suspended over a vertical shaft or cliff whereby therequired tension is supplied. Such a system would employ the force ofgravity to supply the stress required to stretch the reinforcing wirestrands similar to that of the incline of the preferred embodiment. Saidweight does not necessarily have to comprise hardened concrete or solidmaterial but may consist, for example, of a container holding water orother liquid of the necessary weight. It is required only, in thisinstance, that control of tension be maintained by a computer programmedto convert the tension-produced data derived from the effect ofgravitational forces on the mass of the concrete and mold forms movingdownwardly into the braking action of retarders while simultaneously andautomatically relating said data to control of reinforcing wire strandsthrough a wire control device whereby a programmed reduction of stressesis obtained according to the size and strength requirements of theparticular article being manufactured and that such a system beinstalled on an endless track as is taught by this invention.

The method of my invention is thus seen to be a procedure for producingprestressed concrete products through the utilization of gravitationalforces produced when the concrete or a similarly placed weight attachedto the reinforcing wire strands is caused to roll or fall undercontrolled conditions from a higher to a lower position with respect tothe center of the earth.

What is claimed is:

1. An improved method for the continuous manufacture of prestressedconcrete articles wherein the improvement comprises the simultaneous andprogressive steps of:

a. continuously feeding reinforcement strands through a brakingmechanism to the first mold in a train of moving mold forms;

b. discharging fluent concrete into said mold forms as the molds areguided along a substantially level portion of an endless track, saidendless track having an inclined portion included therein;

c. moving said mold;forms along said substantially level-portion of saidendless track to a point at which the concrete becomes hard;

d. advancing said mold forms containing said hardened concrete alongsaid inclined portion exclusively by forceof gravity whereby said moldforms descend said inclined portion thereby loading said reinforcementstrands to provide the required stress;

e. transmitting a gradual prestressing force to said hardened concreteby computer-controlled means by gradually reducing the stressing load onsaid reinforcement strands;

f. correlating the speed of said moving mold forms with the tension onsaid reinforcement strands whereby control is maintained of theprestressing force transferred to said concrete articles;

g. maintaining constant spacing between said mold forms to a point atwhich the concrete becomes hard; and

h. gradually reducing the prestressing forces on the strands subsequentto the hardening of said concrete.

2. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein said gradualprestressing force comprises the force supplied by said mold formshaving hardened concrete therein and wherein said mold forms advancealong said inclined portion exclusively by force of grav ity.

3. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein said computer-controlledmeans for transmitting said prestressing force to said reinforcementstrands comprises a computer programmed to convert tension-producingdata created by the weight of said mold cars moving down said inclinedportion into the braking action required to maintain a preset tension onsaid reinforcement strands.

4. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein said gradual reductionof prestressing force is accomplished through the use of a series ofbrakes of retarders on said inclined portion of said endless track.

5. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein said endless trackhaving an inclined portion included therein utilizes the topography ofhilly or mountainous terrain.

6. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein said inclined portion ofsaid endless track forms an angle of between 10 and from the horizontal.

7. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 including utilizing wire supplymeans for continuously feeding reinforcement strands into said movingmold forms, attachment means for fastening wires to the first mold formin the series and braking means for providing tension upon the moldforms in series until said fluent concrete has hardened and said moldforms have reached said inclined portion.

8. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 1 wherein utilizing means forcorrelating the speed of said moving mold forms with the tension on saidreinstrands.

9. An improved method for the continuous manufacture of prestressedconcrete articles as recited in claim 8 wherein said computerizedutilization of tensionproduced data and stress tables to automaticallyand simultaneously control said braking mechanism comprises varying thedegree of slope of the incline as performed by said retarding means.

1. AN IMPROVED METHOD FOR THE CONTINUOUS MANUFACTURE OF PRESTRESSEDCONCRETE ARTICLES WHEREIN THE IMPROVEMENT COMPRISES THE SIMULTANEOUS ANDPROGRESSIVE STEPS OF: A. CONTINOUSLY FEEDING REINFORCEMENT STRANDSTHROUGH A BARKING MECHANISM TO THE FIRST MOLD IN A TRAIN OF MOVING MOLDFORMS, B. DISCHARGING FLUENT CONCRETE INTO SAID MOLD FORMS AS THE MOLDSARE GUIDED ALONG A SUBSTANTIALLY LEVEL PORTION OF AN ENDLESS TRACK, SAIDENDLESS TRACK HAVING AN INCLINED PORTION INCLUDED THEREIN, C. MOVINGSAID MOLD FORMS ALONG SAID SUBSTANTIALLY LEVEL PORTION OF SAID ENDLESSTRACK TO A POINT AT WHICH THE CONCRETE BECOMES HARD, D. ADVANCING SAIDMOLD FORM CONTAINING SAID HARDENED CONCRETE ALONG SAID INCLINED PORTIONEXCLUSIVELY BY FORCE OF GRAVITY WHEREBY SAID MOLD FORMS DESCEND SAIDINCLINED PORTION THEREBY LOADING SAID REINFORCEMENT STRANDS TO PROVIDETHE REQUIRED STRESS, E. TRASMITTING A GRADUAL PRESTRESSING FORCE TO SAIDHARDENED CONCRETE BY COMPUTER-CONTROLLED MEANS BY GRADUALLY REDUCING THESTRESSING LOAD ON SAID REINFORCEMENT STRANDS, F. CORRELATING THE SPEEDOF SAID MOVING MOLD FORMS WITH THE TENSION ON SAID REINFORCEMENT STRANDEWHEREBY CONTROL IS MAINTAINED OF THE PRESTRESSING FORCE TRANSFERRED TOSAID CONCRETE ARTICLES, G. MAINTAINING CONSTANT SPACING BETWEEN SAIDMOLD FORMS TO A POINT AT WHICH THE CONCRETE BECOMES HARD, AND H.GRADUALLY REDUCING THE PRESTRESSING FORCES ON THE STRANDS SUBSEQUENT TOTHE HARDENING OF SAID CONCRETE.
 2. An improved method for the continuousmanufacture of prestressed concrete articles as recited in claim 1wherein said graDual prestressing force comprises the force supplied bysaid mold forms having hardened concrete therein and wherein said moldforms advance along said inclined portion exclusively by force ofgravity.
 3. An improved method for the continuous manufacture ofprestressed concrete articles as recited in claim 1 wherein saidcomputer-controlled means for transmitting said prestressing force tosaid reinforcement strands comprises a computer programmed to converttension-producing data created by the weight of said mold cars movingdown said inclined portion into the braking action required to maintaina preset tension on said reinforcement strands.
 4. An improved methodfor the continuous manufacture of prestressed concrete articles asrecited in claim 1 wherein said gradual reduction of prestressing forceis accomplished through the use of a series of brakes of retarders onsaid inclined portion of said endless track.
 5. An improved method forthe continuous manufacture of prestressed concrete articles as recitedin claim 1 wherein said endless track having an inclined portionincluded therein utilizes the topography of hilly or mountainousterrain.
 6. An improved method for the continuous manufacture ofprestressed concrete articles as recited in claim 1 wherein saidinclined portion of said endless track forms an angle of between 10* and60* from the horizontal.
 7. An improved method for the continuousmanufacture of prestressed concrete articles as recited in claim 1including utilizing wire supply means for continuously feedingreinforcement strands into said moving mold forms, attachment means forfastening wires to the first mold form in the series and braking meansfor providing tension upon the mold forms in series until said fluentconcrete has hardened and said mold forms have reached said inclinedportion.
 8. An improved method for the continuous manufacture ofprestressed concrete articles as recited in claim 1 wherein utilizingmeans for correlating the speed of said moving mold forms with thetension on said reinforcement strands comprises computerized utilizationof tension-produced data and stress tables to automatically andsimultaneously control braking mechanisms linked to said mold forms andsaid reinforcement strands.
 9. An improved method for the continuousmanufacture of prestressed concrete articles as recited in claim 8wherein said computerized utilization of tension-produced data andstress tables to automatically and simultaneously control said brakingmechanism comprises varying the degree of slope of the incline asperformed by said retarding means.